(1) Field of the Invention
The present invention generally relates to a data readout system applied to optical disk units, such as magento-optical disk units, and more particularly to a data readout system for reproducing, in accordance with a maximum likelihood data detection method, data from an optical disk in which data has been recorded based on a partial response characteristic.
(2) Description of Related Art
Due to large capacity, changeability, high reliability and the like, popularization of magneto-optic disk units is accelerated so that units capable of recording and reproducing image data and units capable of recording codes for computers have been proposed.
A recording system of a conventional optical disk unit is essentially formed as shown in FIG. 1. Referring to FIG. 1, there are provided an optical disk 1, an optical head 2, a data output unit 3, a modulator 4 and a laser driving unit 5. Data from the data output unit 3 is modulated by the modulator 4, and the modulated signal is supplied to the laser driving unit 5. The laser driving unit 5 then drives a laser diode (LD) in the optical head 2 based on the modulated signal, so that data is recorded in the optical disk 1. In a recording process, data, a laser driving signal and pits formed on the optical disk are shown, for example, in FIG. 2. That is, pits are formed at positions at which the laser diode is turned on in accordance with the recording data. In addition, in a case where a magentooptical disk is used, magnetic domains are formed as pits on the magneto-optical disk.
A reproduction system is essentially formed as shown in FIG. 3. Referring to FIG. 3, there are provided the optical disk 1, the optical head 2, an amplifier 6, a filter/equalizer 7, a peak detector 8, a phase locked loop (PLL) circuit 9 and a demodulator 10. The laser diode in the optical head 2 is turned on with a power Pr so that a laser beam is irradiated on the optical disk 1. Based on a reflected beam obtained from the optical disk 1, a reproduction signal as shown in FIG. 4 is obtained. The reproduction signal passes through the aplifier 6, and waveform shaping of the reproduction signal is carried out by an automatic gain controller (AGC), a low-pass filter (LPF) and an equalizer (Eq). After this, the reproduction signal is supplied to the peak detector 8. The peak detector 8 differentiates the reproduction signal, detects zero-cross points of the differential signal so as to detect peak points of the reproduction signal. The peak detector 8 then outputs a pulse signal referred to as row data. The row data is supplied to the PLL circuit 9 and separated data synchronized with a clock is output from the PLL circuit 9. The separated data is demodulated to the original recording data by the demodulator 10.
In a case where data has been recorded in a low recording density, the reproduction signal as shown in FIG. 5A is obtained. In a case where data has been recorded in a high recording density, the reproduction signal as shown in FIG. 5B is obtained. Noises caused by the laser diode (LD) and the optical disk 4 and noises generated in circuits are superposed on the waveform of the reproduction signal. The higher the data recording density on the optical disk 1, the smaller the amplitude of the reproduction signal due to the heat interference in a recording process and the waveform interference in a reproduction process. As a result, in the conventional detecting system, a detecting margin is lowered, so that errors occur easily.